Finding the perfect partner: TLC and FAPA make a lovely couple

Ezine

Published: Feb 15, 2016

Author: Jon Evans

Channels: Detectors

Plasma afterglow

Thin-layer chromatography (TLC) and ambient mass spectrometry may be made for each other, but which specific ambient mass spectrometry technique offers the best match? Already, techniques such as direct analysis in real time (DART), which ionizes samples by firing a stream of plasma at them, and desorption electrospray ionization (DESI), which ionizes samples by firing a fine spray of charged water molecules at them, have been combined with TLC to great effect.

Now, though, a team of Polish scientists have found another potential partner for TLC, in the form of an ambient mass spectrometry technique known as flowing atmospheric pressure afterglow (FAPA). And this could really be a match made in heaven, producing an analytical system that is cheaper, faster and boasts a higher resolution than other combinations, even with a chaperone hanging around as well.

Rather than use a plasma to ionize samples, FAPA uses the afterglow that remains after the plasma can no longer be sustained. In FAPA, a flow of helium atoms is converted into a plasma by an electromagnetic field inside an ion source, producing a plasma stream that flows out of the ion source. Once outside, the electromagnetic field is no longer strong enough to maintain the plasma and so it turns into a stream of radiation known as afterglow, which is still charged enough to ionize any molecules that it encounters.

Laser pointer

The main problem in using FAPA with TLC is that the stream of afterglow is not powerful enough on its own to desorb analytes from a TLC plate and then ionize them. So the Polish team, led by Michał Cegłowski at Adam Mickiewicz University in Poznan, decided to try using a diode laser pointer, costing just €150, to desorb the analytes from the TLC plate.

The system they came up with comprises a TLC plate mounted on a moveable stage, with the FAPA ion source placed at right angles to the stage and around 1cm above it, such that the afterglow flows across the surface of the plate. After using TLC to separate a sample into distinct spots, the TLC plate is placed on the moveable stage. Each sample spot is then moved under the afterglow stream in turn and irradiated with the laser, causing analytes in each spot to desorb from the plate and enter the stream, where they are ionized. The stream then carries the ionized analytes to the entrance of a mass spectrometer, which is placed opposite the ion source on the other side of the moveable stage.

When Cegłowski and his team first tried using the laser to desorb analytes from sample spots, however, they found that the white TLC plate scattered the laser beam, reducing its power so that it was unable to desorb the analytes. To solve this problem, they simply drew a thin pencil line on the TLC plate in front of the samples; this line was sufficiently dark to absorb the beam, preventing it from being scattered. The laser beam was then left on while the plate was moved, with the beam burning a dark track into the plate that prevented any further scattering as each spot was moved under it.

Low cost and high speed

With this problem solved, Cegłowski and his team next tested this combination of TLC and FAPA on a mixture of five pyrazole derivatives, a mixture of nicotine and sparteine, and an extract from a drug tablet containing paracetamol, propyphenazone and caffeine. They found that each of the mixtures were well separated by TLC, producing distinct sample spots that generated high quality spectra when irradiated by the laser and ionized by FAPA, sufficient for each of the analytes to be identified from the spectra. Using one of the pyrazole derivatives as a representative analyte, they then determined the detection limit of this system as 35ng/cm2 (or 10ng per spot), making it just as sensitive as other combinations of TLC and ambient mass spectrometry.

Where this system has the edge over other combinations, though, is in its low cost and high speed, taking just three seconds to produce the spectra for each spot. What is more, Cegłowski thinks there’s great potential for shrinking this combination of TLC and FAPA to produce a single instrument, suggesting that this could be a relationship that lasts for some time.